Add license header to all files

[odow]

Closes #1812

Header taken from https://opensource.google/documentation/reference/releasing/licenses#mit_header

Script was:

HEADER = """
# Copyright (c) 2017: Miles Lubin and contributors
# Copyright (c) 2017: Google Inc.
#
# Use of this source code is governed by an MIT-style license that can be found
# in the LICENSE.md file or at https://opensource.org/licenses/MIT.

"""
for (root, dirs, files) in walkdir("/Users/oscar/.julia/dev/MathOptInterface")
    for file in files
        if endswith(file, ".jl")
            filename = joinpath(root, file)
            contents = read(filename, String)
            write(filename, HEADER * contents)
        end
    end
end